Metamorphism is the geological process that transforms existing rocks through changes in temperature, pressure, or chemically active fluids. This transformation occurs in the solid state, meaning the rock never fully melts, leading to a new mineral composition and texture. Basalt, a common, dark, fine-grained volcanic rock, serves as an excellent starting material, or protolith, for metamorphism due to its widespread presence in the Earth’s crust. The effects of metamorphism on basalt are well-studied because its chemical makeup dictates a predictable sequence of mineral changes under varying conditions, providing geoscientists with a record of the specific conditions the rock experienced.
Understanding Basalt: The Protolith
Basalt is a mafic igneous rock, indicating it is rich in iron (Fe) and magnesium (Mg) oxides and relatively low in silica (SiO₂). This specific chemistry fundamentally controls the types of new minerals that form during metamorphism. The primary original minerals in basalt are calcium-rich plagioclase feldspar and pyroxene, often in the form of augite. These minerals crystallized at high temperatures when the lava rapidly cooled near the Earth’s surface.
The fine-grained texture of basalt, known as aphanitic, results from this rapid cooling process. The high concentration of calcium, magnesium, and iron defines the mafic composition. This chemical signature ensures that the new minerals formed during metamorphism will also be iron- and magnesium-bearing silicates, distinctly different from the products of metamorphosed silica-rich rocks like granite.
The Environmental Triggers of Transformation
The transformation of basalt requires geological triggers to reorganize its existing mineral structure. Heat, which increases with depth along the geothermal gradient or is supplied by a magmatic intrusion, is one such trigger. Temperatures exceeding about 150°C are required to initiate the chemical reactions that define metamorphism. This increase in temperature makes the original basaltic minerals unstable and promotes the growth of new, stable mineral phases.
Pressure is another primary agent, arising either from the weight of overlying rock (lithostatic pressure) or from tectonic forces (differential stress). Differential stress, common in mountain-building events, causes the rock to deform and often results in a layered or foliated texture. Furthermore, chemically active fluids, such as hot, circulating seawater or magmatic water, can significantly accelerate the process. These hydrothermal fluids, rich in dissolved ions, facilitate the necessary chemical reactions by transporting material and allowing rapid mineral recrystallization, a process common in basalt along mid-ocean ridges.
The Metamorphic Facies Sequence
Metamorphism of basalt follows a predictable sequence of mineral transformations as pressure and temperature conditions increase, defined by distinct groupings of minerals known as metamorphic facies. The mineral assemblage present in the final rock acts as a precise indicator of the pressure and temperature conditions under which it formed.
Greenschist Facies
The greenschist facies represents low-grade metamorphism, typically occurring between 300°C and 450°C. Under these conditions, the original calcium-rich plagioclase and pyroxene break down and recrystallize into a characteristic green assemblage. The newly formed minerals include chlorite, epidote, and actinolite, which imparts a distinctive green color to the resulting rock, aptly named greenschist. The formation of these hydrous minerals involves incorporating water into the crystal structure.
Amphibolite Facies
As temperature and pressure rise, the rock enters the amphibolite facies. The minerals stable in the greenschist phase, such as chlorite and actinolite, become unstable and react to form the more temperature-resistant amphibole mineral, hornblende. The rock formed is called amphibolite, and its mineral makeup consists primarily of hornblende and plagioclase feldspar. This transformation is marked by the loss of water from the hydrous greenschist minerals as the rock adjusts to the hotter environment.
Eclogite Facies
The most extreme conditions are represented by the eclogite facies, characterized by very high pressures and moderate temperatures, often associated with deep burial in subduction zones. In this environment, the dense mineral assemblage forms directly from the basaltic protolith. The plagioclase and pyroxene completely reorganize to create a mixture of garnet and a specific sodium-rich pyroxene called omphacite. This mineral combination results in an exceptionally dense rock, and the breakdown of hydrous minerals releases significant amounts of water, which can influence overlying mantle rock.